1. This invention relates to the manufacture of alloy powder, and, more particularly, to the manufacture of superalloy or titanium alloy powders characterized by reduced amounts of impurities.
2. Description of the Prior Art
A wide variety of alloy powder manufacturing methods and apparatus are well known in the metallurgical art. As such manufacture relates to high temperature alloys and superalloys, for example the type based on Fe, Co, Ni, Ti or their combinations, current powder production methods include first melting the alloy elements in a high vacuum furnace chamber through use of vacuum electron beam, vacuum arc, vacuum induction or inert gas plasma melting to produce an ingot. After production of the alloy ingot, current powder production techniques convert the alloy ingot into powder by such methods as gas atomization, rotary atomization and vacuum atomization utilizing ceramic, graphite, or refractory hearth primary melting in conjunction with a ceramic, graphite, or refractory tundish and nozzle for producing a liquid metal stream needed to produce powder.
Certain high temperature operating and highly stressed components of gas turbine engines, for example, turbine disks, use powder metal in their manufacture. By producing a powder metal preform nearly to the final shape of the component, manufacturing costs can be reduced. Alternatively, an intermediate shape can be produced and then later processed to the final form. Alloys produced by powder metallurgical techniques exhibit a uniform microstructure and minimal chemical segregation, yielding a consistent product with a high degree of workability. However, it has been recognized that inadequate powder cleanliness, particularly from ceramic particles introduced in currently used powder manufacturing processes, can result in a significant reduction in mechanical properties such as low cycle fatigue in the finished component. This reduction is due to the presence in the consolidated powder metal disks of inclusions which act as initiation sites for low cycle fatigue failures. Nearly all superalloy powder metal for such applications currently is produced by first providing an ingot, melting the ingot and then making powder by gas atomization processes. Such atomization processes utilize ceramic melting and pouring devices, and it has been found that these devices introduce a significant proportion of the undesirable ceramic inclusions. It should be recognized that the present invention can be particularly useful when the starting materials are relatively free of such ceramic inclusions.